Copper nanoparticle application processes for low temperature printable, flexible/conformal electronics and antennas

ABSTRACT

An ink adapted for forming conductive elements is disclosed. The ink includes a plurality of nanoparticles and a carrier. The nanoparticles comprise copper and have a diameter of less than 20 nanometers. Each nanoparticle has at least a partial coating of a surfactant configured to separate adjacent nanoparticles. Methods of creating circuit elements from copper-containing nanoparticles by spraying, tracing, stamping, burnishing, or heating are disclosed.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application is related to U.S. application Ser. No.12/512,315, filed Jul. 30, 2009 and currently pending.

Statement Regarding Federally Sponsored Research or Development

Not Applicable.

BACKGROUND

1. Field

The present invention relates to printed electronics and, in particular,the creation of circuit elements using copper nanoparticles.

2. Description of the Related Art

Electrical assemblies are currently fabricated using a rigid substratewith individual components attached to the substrate and interconnectedwith conductive paths or “traces” on the substrate. The traces aretypically created on the surface of the substrate by coating the entiresurface of the substrate with a layer of copper, masking the copper withthe interconnect pattern using a photolithographic process, andselectively etching away the non-masked copper. The minimum separationof the traces is often limited by the etching process. More complicatedcircuits are fabricated using multiple layers of circuit tracesseparated by insulating layers with connections between the conductivelayers formed by holes between the insulating layers that are filledwith conductive material. These interlayer connections are called“vias.” A rigid substrate with one or more layers of circuit traces isreferred to as a printed circuit board (PCB) and an electronic assemblythat is formed by mounting electronic components to the PCB is a printedcircuit board assembly (PCBA).

The drive to fit electronics into smaller or curved packages drove thedevelopment of flexible substrates where the traces are created byplating and etching as done with the rigid substrates or screen printinga conductive material onto the flexible substrate. These flexibleprinted circuits (FPCs) are limited in the separation of circuitelements, referred to as the “pitch” of the traces, in the same way asconvention rigid PCB fabrication as they use the same processes to formthe circuits.

The ability to directly print circuit elements has been developed in thelast decade or so to take advantage of low-cost printing technologies.Common printing processes such as screen printing, gravure, offsetlithography, and inkjet have been used to create circuits using bothconductive carbon-based compounds and metals. Each of the processes haveadvantages and disadvantages related to resolution, throughput, andcost. Circuits fabricated from carbon-based compounds have a lowerconductivity than metal circuits. The metal inks require temperatures ofup to 300° C. to fuse the metal particles into a continuous conductivestrip, limiting the substrate to materials that are stable at thistemperature.

It would be beneficial to be able to produce highly conductive circuitsand circuit elements on both rigid and flexible substrates with a finerpitch than possible with current printing technologies and/or withoutrequiring elevated process temperatures.

SUMMARY

The present invention includes the printing of electronic circuits andelements using copper nanoparticles, which enables the creation ofcopper circuits and elements onto a variety of rigid and flexiblesubstrates at pitches below 100 micrometers. When nanoparticles having adiameter of under 20 nanometers, preferably under 10 nanometers, morepreferably in the range of 1-7 nanometers, and even more preferably inthe range of 3-5 nanometers, are printed onto a substrate in a mannersimilar to that of inkjet printers, the nanoparticles fuse upon impactwith the substrate. Copper nanoparticles of these sizes can also beapplied in a pattern and fused by exposure to a short-duration pulse ofradiant energy, such as a laser or a bright light, or by exposure to atemperature of less than 200° C., and preferably less than 70° C. Coppernanoparticles of these sizes can also be fused by pressure such ascompression under a form or by tracing the desired pattern with amechanical stylus such as a nanoinscriber. Forming circuit elements inthe methods described herein allows the use of substrate materials, andparticularly flexible materials, that cannot tolerate the chemicals andtemperatures of the current processes used to create circuit elements onsubstrates for electronic assemblies. The methods of printing andforming circuit elements from copper nanoparticles as described hereinalso enable finer pitch circuits, i.e. having small separation distancesbetween conductive elements, than possible with other processes. Circuitelements formed from copper nanoparticles may include passive devicessuch as resistors, capacitors, and inductors, active devices such astransistors, Radio Frequency (RF) elements such as antennae, reflectors,and waveguides, other circuit elements such as ground and power planes,shielding, and signal paths, and even complete devices such as a RadioFrequency IDentification (RFID) tag.

In certain embodiments, a circuit element is disclosed that comprises afirst layer of formed metal comprising fused nanoparticles that comprisecopper and had a diameter of less than 20 nanometers prior to beingfused.

In certain embodiments, a circuit assembly is disclosed that comprises asubstrate and a first layer of formed metal coupled to the substrate,the first layer of formed metal comprising fused nanoparticles thatcomprise copper and had a diameter of less than 20 nanometers prior tobeing fused.

In certain embodiments, a circuit-printing device is disclosed thatcomprises a sprayer configured to emit a plurality of drops of a mixturecomprising nanoparticles that comprise copper and have a diameter ofless than 20 nanometers toward a substrate with sufficient velocity thatthe nanoparticles fuse with each other upon impact with the substrateand form a plurality of dots on the substrate, wherein each dotcomprises a layer of fused nanoparticles and overlapping dots are fusedto each other.

In certain embodiments, a method of creating a conductive element on asubstrate is disclosed. The method comprising the step of spraying aplurality of drops of a mixture comprising nanoparticles that comprisecopper and have a diameter of less than 20 nanometers toward a substratewith sufficient velocity that the nanoparticles fuse with each otherupon impact with the substrate and form a plurality of dots on thesubstrate, wherein each dot comprises a layer of fused nanoparticles andoverlapping dots are fused to each other.

In certain embodiments, a method of creating a conductive element on asubstrate is disclosed. The method comprising the steps of applying alayer of a mixture comprising nanoparticles that comprise copper andhave a diameter of less than 20 nanometers over at least a portion of asurface of a substrate, and fusing the nanoparticles together in atleast a portion of the mixture layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustrative example of a printed circuit assembly (PCA)having a flexible substrate onto which electrical components andcircuits may be printed according to certain aspects of the presentdisclosure.

FIG. 2 depicts an exemplary method of printing a circuit elementaccording to certain aspects of the present disclosure.

FIGS. 3A and 3B depict a portion of a PCA created by printing accordingto certain aspects of the present disclosure.

FIG. 4 depicts a resistor formed by printing and fusing coppernanoparticles according to certain aspects of the present disclosure.

FIG. 5 depicts a capacitor formed by printing and fusing coppernanoparticles according to certain aspects of the present disclosure.

FIGS. 6A and 6B depict an inductor formed by printing and fusing coppernanoparticles according to certain aspects of the present disclosure.

FIGS. 7A and 7B depict an active device formed by printing and fusingcopper nanoparticles according to certain aspects of the presentdisclosure.

FIG. 8 depicts a method of fusing nanoparticles using a laser accordingto certain aspects of the present disclosure.

FIGS. 9A-9D depicts a method of fusing nanoparticles by compressing thenanoparticles using a form according to certain aspects of the presentdisclosure.

FIG. 10 depicts a method of fusing nanoparticles by nanoinscribingaccording to certain aspects of the present disclosure.

FIG. 11 depicts a method of applying a patterned layer of a mixturecomprising copper nanoparticles according to certain aspects of thepresent disclosure.

FIGS. 12A-12D depict methods of fusing a patterned layer of a mixturecomprising copper nanoparticles according to certain aspects of thepresent disclosure.

FIG. 13 depicts a method of fusing a layer of a mixture comprisingcopper nanoparticles by using a roller according to certain aspects ofthe present disclosure.

FIGS. 14A-14C depict methods of fusing a layer of a mixture comprisingcopper nanoparticles in a passage according to certain aspects of thepresent disclosure.

FIGS. 15A-15B depict another method of printing a circuit elementaccording to certain aspects of the present disclosure.

DETAILED DESCRIPTION

Some inkjet printers use a piezoelectric actuator to eject a drop of inkfrom an ink-filled chamber or channel beneath each nozzle. When avoltage is applied, the piezoelectric material changes shape, creating apressure wave in the channel that ejects a droplet of ink from thenozzle with sufficient velocity to reach the paper on which the image isbeing created. Piezoelectric inkjet technology allows the use of a widervariety of inks than thermal inkjet technology as there is norequirement for a volatile component, and no issue with kogation of theink. One printing process is referred to as “drop-on-demand”, whereinsoftware directs the printheads to eject one or more droplets of inkfrom a nozzle only when it is desired to create an image on the portionof the paper immediately in front of the nozzle.

Copper particles having diameters of less than 20 nanometers haveproperties that are not linear extensions of their bulk properties. Atthese diameters, copper nanoparticles are considered to be “meta-stable”and require very little energy to mobilize the atoms to fuse whennanoparticles are in direct contact. The energy required decreasesnonlinearly as diameters of the nanoparticle decreases, and reaches alevel in the range generally below 10 nanometers where nanoparticlesfuse upon direct metal-to-metal contact. Copper nanoparticles must becoated with a surfactant to prevent the nanoparticles from fusing whendispersed in a liquid. A mixture of the surfactant-coated nanoparticleswith a carrier liquid can be treated in much the same way that an ink istreated, except the copper-containing mixture requires certainadditional conditions to fuse the copper nanoparticles at the desiredtime. Copper nanoparticles of less than 20 nanometers appear to have acolor ranging from red to black, depending on the degree ofagglomeration and aggregation, facilitating the absorption of radiantenergy. One of the advantages of creating circuit elements or otherconductive structures is that the energy required to fuse the coppernanoparticles is so low that the substrate temperature does not increaseabove 70° C. and, in certain embodiments, is barely heated at all. Thiscompares to current processes of fusing gold and silver nanoparticles inovens at temperatures above 70° C. and up to 300° C. for periods of timethat may be as long as 30 minutes.

FIG. 1 is an illustrative example of a PCA 10 having a flexiblesubstrate 16 onto which electrical components 14 and circuits 12 may beprinted according to certain aspects of the present disclosure. Simpleelectrical circuits have been previously created on flexible substratessuch as polyimide that are able to withstand the chemicals used to plateand etch the circuits and survive exposure to temperatures over 200° C.One advantage of flexible PCAs is that electronics built on the flexiblePCAs may be packaged into small and irregularly shaped spaces. Aflexible substrate also allows a PCA to conform to a curved supportstructure such that heat can be more effectively transferred to thestructure. The difficulty in getting the heat out of electronics builton rigid printed circuit boards (PCBs) limits the functionality of theelectronics. Printing the circuits 12 and components 14 onto a substrate16 using a process that does not require high temperature expose opensup a variety of substrate materials that are otherwise excluded by thelow-temperature limitation of the material.

FIG. 2 depicts an exemplary method of printing a circuit elementaccording to certain aspects of the present disclosure. An inkjetprinter 29, or similar printing device, includes a printhead 30 that hasa nozzle 32 connected to an ink manifold 38. In this embodiment, apiezoelectric driving element 36 intrudes into the nozzle 32 such thatin the non-energized configuration, the driving element 36 retracts tothe left, in the orientation of FIG. 2, and connects the ink manifold 38to the nozzle 32. Ink flows from the ink manifold 38 into the nozzle 32and partially fills the nozzle 32. When it is desired to eject a drop ofink, a voltage is applied to the piezoelectric driving element 36 whichexpands toward the right, in the orientation of FIG. 2. This expansioninto the nozzle 32 ejects a drop 20.

In the embodiment of FIG. 2, the “ink” is a mixture that includes aplurality of nanoparticles 22 that comprise copper. In certainembodiments, the size of these nanoparticles 22 is less than 20nanometers, preferably less than 10 nanometers, more preferably between1-7 nanometers, and even more preferably between 3-5 nanometers.Although the nanoparticles 22 are not necessarily true spheres, thenanoparticles 22 are considered to have a “size” or “diameter” that isan average of multiple surface-to-surface distances, measured throughthe center of mass of the nanoparticle 22, over a variety of positionsabout the nanoparticle 22. Nanoparticles 22 that are not near-sphericalstill exhibit some of the properties described herein but the behaviorof the particles is not as uniform and consistent as it is fornear-spherical nanoparticles. In certain embodiments, the mixtureincludes a liquid in which the nanoparticles 22 are dispersed. Incertain embodiments, the liquid may comprise one or more of water,alcohol, solvents, or other carrier materials and may further comprisesurfactants, dispersants, stabilizers, or other chemicals that areapplied to the surface of the nanoparticles 22 or dissolved in thesolvent to maintain particle separation and avoid clumping or adhesionof the nanoparticles 22 in the mixture or modify the viscosity, surfacetension, or other attribute of the mixture. In certain embodiments, thenanoparticles 22 comprise at least a partial coating of a surfactant(not visible in FIG. 2).

One of the attributes of copper nanoparticles 22 that are 20 nanometersor less in size, preferably less than 10 nanometers, more preferablybetween 1-7 nanometers, and even more preferably between 3-5 nanometers,is that the nanoparticles 22 will fuse upon metal-to-metal contact witheach other. One way to keep that nanoparticles 22 from fusing in themixture prior to printing is to coat each nanoparticle 22 with a layerof a surfactant or other material that tends to space apart or repulseother nanoparticles 22 that are coated with the same material.

When a drop 20 of the mixture is ejected from the nozzle 32 of theprinter 29, it travels the open space between the printer 29 and thesubstrate 28 and strikes the substrate 28 with a velocity that mayexceed 10 meters per second (22 miles per hour). When the drop 20strikes the substrate 28, the nanoparticles 22 tend to move through theliquid in the former direction of the drop 20. As the firstnanoparticles 22 stop upon contact with the substrate 28, additionalnanoparticles 22 will continue to move through the liquid and strike thestationary nanoparticles 22 with sufficient energy to displace thecoating of surfactant of the two nanoparticles 22 and allowmetal-to-metal contact between the nanoparticles 22. Collision-inducedreactions at the molecular level have been dubbed “chemistry with ahammer.” The nanoparticles 22 must have enough energy to push aside thesurfactants on the surface of the colliding nanoparticles 22 such thatthe metal cores of the two nanoparticles 22 make contact. Thismetal-to-metal contact allows the reactive surface atoms to startflowing, thus causing fusion bonding between the nanoparticles 22 intolarger crystals and solid layers. As more nanoparticles 22 of the drop20 of the mixture strike the growing fused mass of nanoparticles 22, alayer of metal 26 is formed as a dot on the substrate 28. This layerwill tend to adhere to the substrate by simple mechanical adhesion, asmost surfaces are irregular at the nanoscale and the fused layer ofmetal will be interlocked with the irregular surface features of thesubstrate 28. FIG. 2 shows a cross-section of substrate 28 wherein adrop 24 has struck the substrate 28 and the nanoparticles 22 havecollided to form a dot of metal 26 on the surface of the substrate 28.

The interparticle pressure required to fuse nanoparticles 22 of about 25nanometers and smaller is in the range of approximately 600-13,800kilopascals (kPa) (roughly 90 to 2000 pounds per square inch (psi)),depending on the size of the nanoparticles 22 and the surfactants thatare included in the mixture. Larger particles tend to be increasing hardto fuse, requiring higher pressures or pressure in conjunction withheat.

In certain embodiments, the liquid in the mixture may be formulated suchthat the liquid in each drop 20 at least partially evaporates during theflight from the printer nozzle 32 to the substrate 28. In certainembodiments, all of the liquid in each drop 20 evaporates leaving onlyindividual nanoparticles 22 to individually strike the substrate andfuse.

FIGS. 3A and 3B depict a portion of a PCA created by printing accordingto certain aspects of the present disclosure. FIG. 3A shows three dots26A, 26B, 26C created by the printing process shown in FIG. 2. Each dot26, in this embodiment, is an irregular circle of metal that remainsafter the carrier liquid of the mixture has evaporated. In otherembodiments, these dots 26 will have an elongated shape caused by thelateral velocity of the substrate 28 passing or under the printer nozzle32. In the embodiment of FIG. 3A, the substrate 28 has shifted relativeto the nozzle 32 between the ejection of sequential drops 20 of mixturesuch that the dots 26A, 26B, 26C formed by the drops 20 of the mixtureare offset in an overlapping pattern.

FIG. 3B is a cross-section side view of the dots 26 and substrate 28 ofFIG. 3A. It can be seen that the layers of metal 26 that form the dotsare overlapping the adjacent layers of metal 26. Some of the firstnanoparticles 22 of the drop of mixture that formed dot 26B that reachedthe substrate 28 struck the metal layer of the previous dot 26A,whereupon the nanoparticles of dot 26B fused with the layer of metal ofdot 26A such that the layers of metal of dots 26A and 26B areelectrically connected. Similarly, the metal layer of dot 26C is fusedwith the metal layer of dot 26B and therefore electrically connected tothe metal layers of both dots 26B and 26C. In this manner, an electricalpathway or circuit is formed by continuously overlapping dots 26 betweenthe two points that are to be electrically connected. This electricalcircuit can be wider than a single row of dots and follow any pattern,limited only by the patterning capabilities of the printer. The printingof electrical circuit and elements can be controlled in a manner similarto that used to print with ink on paper, wherein any shaped or patternedmetal layer can be formed in the same manner that an image can beprinted on a standard printer.

The printing method of FIGS. 2 and 3A-3B can be used to createmultilayer PCAs. The first layer of circuitry is formed by printing onthe bare substrate, forming a layer of formed metal. A layer of anonconductive material is then applied over a portion of the first layerof formed metal. In certain embodiments, this nonconductive layer isapplied by a similar printing process using a nonconductive mixture thathardens or cures after application. A second layer of formed metal isthen created by printing over the first layer of formed metal and thelayer of nonconductive material. Where the second layer of formed metaloverlaps exposed areas of the first layer of formed metal, an electricalconnection between the layers is formed. This is discussed in greaterdetail with respect to FIG. 6B.

FIG. 4 depicts a resistor 30 formed by printing and fusing coppernanoparticles 22 according to certain aspects of the present disclosure.The resistor 30 is formed in a single layer of metal 26. The resistanceis created by forming a long, thin path 31 of conductive metal that, inthe embodiment of FIG. 4, follows a serpentine path to create themaximum length of path 31 in a minimum area.

FIG. 5 depicts a capacitor 32 formed by printing and fusing coppernanoparticles 22 according to certain aspects of the present disclosure.This embodiment of a capacitor 32 is formed in a single layer of metal26. Two electrodes 34A and 34B are created with a narrow space 36separating the electrodes 34A, 34B. In certain embodiments, theelectrodes 34A, 34B are created in separate layers with one electrodeabove the other.

FIGS. 6A and 6B depict an inductor 38 formed by printing and fusingcopper nanoparticles 22 according to certain aspects of the presentdisclosure. FIG. 6A is a perspective view of the conductive portions ofthe inductor 38 and FIG. 6B is a cross-section of the inductor 38 takenacross a portion of the inductor 38 passing through the center of theinductor 38. In the embodiment of FIGS. 6A and 6B, a first layer ofmetal 26 forms a spiral 38A. A layer 40 of nonconductive material (notshown in FIG. 6A) is formed over the spiral 38A. A second layer of metal26 is formed as a strip 38B above nonconductive layer 40, with the endof strip 38B over the center of spiral 38A. A via 42 (shown as line ofconnection 42A in FIG. 6A) is formed through the nonconductive layer 40and electrically connects strip 38B to spiral 38A. In certainembodiments, the via 42 is formed by creating a hole through layer 40 tospiral 38A using a process such as laser drilling prior to forming thestrip 38B such that metal 26 fills the hole to create via 42 at the sametime as the strip 38B is formed. In certain embodiments, a hole iscreated after strip 38B is formed, the hole passing through strip 38Band layer 40 to spiral 38A, and the hole is filled with metal 26 in asubsequent operation. In certain embodiments, the printer 29 adjusts theprinting operation to deposit extra drops of mixture over the hole inwhich the via 42 is to be formed so as to provide sufficientnanoparticles 22 to fill the via 42. In certain embodiments, the via 42can be sized and the mixture of nanoparticles 22 formulated to takeadvantage of surface tension to enhance distribution and spreading ofthe mixture within the hole.

It will be apparent that these same techniques can be used to createthree-dimensional structures for other functions such as providingthermally conductive paths for thermal management, for exampleconducting heat from an active electrical component to a heat sink.

FIGS. 7A and 7B depict an active device 50 formed by printing and fusingcopper nanoparticles 22 according to certain aspects of the presentdisclosure. In this embodiment, a first electrode 52 is printed as arectangle onto a substrate 28 (not shown in FIG. 7A). In otherembodiments, the first electrode 52 has other shapes including a circle.A layer of non-conducting material 40 is formed over a portion of firstelectrode 52. In this embodiment, a layer of a semiconductor 56 isapplied over a portion of the first electrode 52. This semiconductor isselected based on the desired function of the active device 50. Incertain embodiments, the semiconductor 56 is adapted for electrontransport. In certain embodiments, the semiconductor 56 is adapted forphoton emission. In certain embodiments, the semiconductor 56 is anorganic material. In this embodiment, a conductor 54 is formed over thenonconductive material 40 and connected to the semiconductor 56. In thisembodiment, a second electrode 58 is formed over a portion of thesemiconductor 56. The first electrode 52, conductor 54, and secondelectrode 58 are connected to other circuit elements that manipulate theactive device 50. It will be apparent to those of ordinary skill in theart that many types of active devices can be constructed usingconductive layers formed from copper nanoparticles 22 as describedherein.

FIG. 8 depicts a method of fusing nanoparticles using a laser 64according to certain aspects of the present disclosure. In thisembodiment, a layer of a mixture 62 containing copper nanoparticles 22having a diameter of less than 20 nanometers, and preferably less than10 nanometers, and more preferably less than 4 nanometers, is spreadevenly across a strip of substrate 28. When copper nanoparticles 22 havea diameter of less than 20 nanometers, the nanoparticles 22 require verylittle energy to fuse. In this embodiment, a laser 64 creates a beam ofoptical radiation 66 and directs this beam to the portions of the layer62 where it is desired to form a pattern 68 of fused nanoparticles. Incertain embodiments, the optical radiation 66 has a defined frequencyband. In certain embodiments, the frequency band of the opticalradiation 66 covers at least a portion of the adsorption band of thenanoparticles 22. The energy of the optical radiation 66 is tuned toprovide just enough energy to fuse the nanoparticles 22 in the layer ofthe mixture 62. In certain embodiments, the energy required to fuse thenanoparticles 22 raises the temperature of the substrate by less than20° C. After the pattern 68 of fused metal is completed, the unfusedmixture 62 is removed, leaving a strip 60 of circuit elements.

FIGS. 9A-9D depicts a method of fusing nanoparticles 22 by compressing,or stamping, the nanoparticles 22 using a form 70 according to certainaspects of the present disclosure. FIG. 9A depicts a layer of mixture 72formed on a substrate 28. A form 70 has a pattern of raised portions 76in the shape of the circuit element to be created. This form is presseddownward under force until it contacts the layer of mixture 72.

FIG. 9B depicts the process after the form 70 has developed pressureagainst layer of mixture 72 and the substrate 28. The nanoparticles 22under the raised portions 76 have been compressed until thenanoparticles 22 are in direct contact with each other. Nanoparticles 22having a diameter in the range of 20 nanometers, preferably less than 10nanometers, more preferably in the range 1-7 nanometers, and even morepreferably in the range 3-5 nanometers, have been shown to fuse underrelatively low pressures and without inducing an increase in thetemperature of the substrate 28. A pressure of 90-2000 psi is required,depending on the size of the nanoparticles 22 and the formulation of themixture 72.

FIG. 9C depicts the process after the form 70 has been removed, leavingbehind a fused pattern of metal 74 that corresponds to the pattern ofthe raised portions 76 of form 70. The portion of the mixture 72 thatwas not compressed has not fused and is removed by a process such asaqueous washing (not shown).

FIG. 9D depicts the finished pattern of metal 74 adhered to substrate28. In certain embodiments, additional components (not shown) areattached to the substrate 28 and in electrical contact with portions ofthe circuit element formed by the pattern of metal 74. In certainembodiments, the circuit element created by the pattern of metal 74 isremoved from the substrate 28 and transferred to a different supportstructure (not shown).

FIG. 10 depicts a method of fusing nanoparticles by nanoinscribingaccording to certain aspects of the present disclosure. Nanoinscribingis the process of tracing the shape of a circuit element with amechanical element 100 over a layer of mixture 72 containingcopper-containing nanoparticles. As previously stated, coppernanoparticles 22 having a diameter in the range of 20 nanometers,preferably less than 10 nanometers, more preferably in the range 1-7nanometers, and even more preferably in the range 3-5 nanometers, can befused by relatively low pressures, for example down to 90 psi, atambient temperatures and without causing an increase in the temperatureof the substrate 28. The mechanical element 100 compresses the mixture72 with sufficient pressure under the tip 102 to fuse the coppernanoparticles 22 in the mixture 72 into a layer of metal 74. Thismechanical element 100, in certain embodiments, has a tip 102 having adiameter on the order of micrometers.

FIG. 11 depicts a method of applying a patterned layer of a mixture 72comprising copper nanoparticles 22 according to certain aspects of thepresent disclosure. In this embodiment, the mixture 72 is applied onlywhere it is intended to create a circuit element by fusing the coppernanoparticles 22. The mixture 72 is applied in a pattern such as simplelines to form conductors or shapes such as the devices of FIGS. 4 and 5can be created. The embodiment of FIG. 11 depicts the mixture 72 beingextruded from the nozzle 112 of a reservoir 110 that is moving over asubstrate 28, as indicated by the arrow 114 that indicates the directionof motion of the reservoir 110. The diameter of nozzle 112 controls thewidth and thickness of the pattern of mixture 72. After the entirepattern of mixture 72 has been formed on the substrate, the entirepattern of mixture 72 is cured by a process such as shown in FIG. 12A.In certain embodiments, the pressure created by the extrusion processinduces at least partial fusion of the nanoparticles 22.

FIGS. 12A-12D depict methods of fusing a patterned layer of a mixture 72comprising copper nanoparticles 22 according to certain aspects of thepresent disclosure. FIG. 12A depicts a patterned layer of mixture 72that has been formed on a substrate 28 being exposed to a light source120 that illuminates an area of the patterned mixture 72 with sufficientenergy that the copper nanoparticles 22 fuse together. In certainembodiments, the light source 120 is energized for less than a second ata first power level. In certain embodiments, the light source 120 isenergized for more than one second at second power level that is lowerthan the first power level. In certain embodiments, the energy deliveredby the light source 120 is sufficient to fuse the copper nanoparticles22 while raising the temperature of the substrate 28 to a temperature ofless than 70° C. In certain embodiments, the energy delivered by thelight source 120 is sufficient to fuse the copper nanoparticles 22 whileraising the temperature of the substrate 28 less than 20° C. In certainembodiments, the radiation emitted from the light source 120 has adefined frequency band. In certain embodiments, the frequency band ofthe radiation emitted from the light source 120 covers at least aportion of the adsorption band of the nanoparticles 22. FIG. 12C depictsthe substrate 28 and fused metal layer 124 that remain after the lightsource 120 has been turned off and any carrier material or unfusedportions of the mixture 72 have been removed by a process such asaqueous washing (not shown).

In certain embodiments, the previously described laser 64 can be used totrace over a pattern of mixture, fusing the nanoparticles 22. Similarly,the nanoinscriber 100 can be used to trace a pattern of mixture to fusethe nanoparticles.

FIG. 12B depicts a patterned layer of mixture 72 that has been formed ona substrate 28 being exposed to a heat source 122 that heats thepatterned mixture 72 with sufficient energy that the coppernanoparticles 22 fuse together. In certain embodiments, the heat source122 is a radiant energy source. In certain embodiments, the heat source122 generates heat that is convectively conveyed to the substrate 28. Incertain embodiments, the energy delivered by the heat source 120 issufficient to fuse the copper nanoparticles 22 while raising thetemperature of the substrate 28 to a temperature of less than 70° C. Incertain embodiments, the energy delivered by the heat source 122 issufficient to fuse the copper nanoparticles 22 while raising thetemperature of the substrate 28 less than 20° C. In certain embodiments,the energy delivered by the heat source 120 is sufficient to fuse thecopper nanoparticles while raising the temperature of the substrate to atemperature of less than 70° C. FIG. 12D depicts the substrate 28 andfused metal layer 124 that remain after the substrate 28 has beenremoved from the heat created by heat source 122 and any carriermaterial or unfused portions of the mixture 72 have been removed by aprocess such as aqueous washing (not shown).

Another method of fusing the nanoparticles 22 shown in FIG. 12A or 12Bis the use of existing vapor-phase reflow process equipment (not shown).As the vapor-phase process liquid is more effective in transferringthermal energy than the air in a convection oven, the temperature of thevapor can be limited to a lower temperature than the air of a convectionoven while still achieving adequate processing speed. This lowertemperature, in conjunction with the lower temperatures required to fusethe nanoparticles 22, may enable fabrication using materials orcomponents that are not suitable for higher-temperature processes.

FIG. 13 depicts a method of fusing a layer of a mixture 72 comprisingcopper nanoparticles by using a roller 130 according to certain aspectsof the present disclosure. In the embodiment of FIG. 13, the roller 130is moved over the substrate 28 at a velocity V, compressing the mixture72 to form a fused pattern of metal 74. In certain embodiments, theroller 132 is rotating at an angular velocity theta (θ) such that theroller surface 132 has a non-zero velocity with respect to the substrate28. In certain embodiments, the roller surface 132 comprises a texturedsurface (not shown). In certain embodiments, the roller surface 132comprises parallel grooves (not shown). In certain embodiments, theroller 130 may be replaced by a ball (not shown) that may rotate withrespect to the substrate 28 or move over the substrate 28 withoutrotation.

FIGS. 14A-14C depict methods of fusing a layer of a mixture 72comprising copper nanoparticles in a passage 134 according to certainaspects of the present disclosure. In certain embodiments, the passage134 is a via and the substrate 28 is a PCB. In FIG. 14A, a layer ofmixture 72 has been formed on an inner surface 135 of the passage 134.In certain embodiments, mixture 72 fills the passage 134. In certainembodiments, mixture 72 fills only a portion of the length of thepassage 134.

FIG. 14B depicts a ball 136 being passed through the passage 134,compressing the mixture 72 to form a fused pattern of metal 74 on theinner surface 135. In certain embodiments, the passage 134 iscylindrical with an inner diameter D1. In the embodiment of FIG. 14B,the ball 136 has a diameter D2 that is less than the passage diameterD1. In certain embodiments, the ball 136 is coupled to a shaft 137. Incertain embodiments, the shaft 137 and ball 136 rotate with respect tothe substrate 28 as the ball 136 passes through the passage 134. Incertain embodiments, the ball 136 is replaced with a cylindrical element(not shown) that may have a rounded, shaped, or pointed lower tip. Incertain embodiments, the ball 138 is replaced with an axially symmetricshaped element (not shown).

FIG. 14C depicts a ball 138 being passed through the passage 134,similar to FIG. 14B, compressing mixture 72 to form a fused pattern ofmetal 74. Ball 138 has a diameter D3 that is greater than or equal tothe passage diameter D1. As ball 138 passes through passage 134, thesubstrate 28 is deformed in the region 140 adjacent to the ball 138. Incertain embodiments, the deformation in region 140 is elastic andsubstrate 28 returns to its original shape after ball 138 has passed. Incertain embodiments, substrate 28 does not fully return to its originalshape. In certain embodiments, ball 137 is coupled to a shaft 137. Incertain embodiments, the shaft 137 and ball 138 rotate with respect tothe substrate 28 as the ball 138 passes through the passage 134. Incertain embodiments, the ball 138 is replaced with a cylindrical element(not shown) that may have a rounded, shaped, or pointed lower tip. Incertain embodiments, the ball 138 is replaced with an axially symmetricshaped element (not shown).

Burnishing the fused pattern of metal 74 may provide one or morebenefits including reduction of fatigue failure, prevention of corrosionor stress corrosion, texturing the surface of the fused pattern of metal74 to eliminate visual defects, reduction of porosity of the fusedpattern of metal 74, and creation of residual compressive stress in thefused pattern of metal 74.

Burnishing by any of ball 136 or 138, roller 130, nanoscribingmechanical element 100, or other contact element may be accomplished ata pressure that the substrate 28 is elastically deformed in the contactarea, returning to the original shape after the contact element movesaway. In certain embodiments, the pressure applied by the contactelement does not cause measureable deformation of the substrate 28.

In certain embodiments, burnishing is performed while the substrate 28and fused pattern of metal 74 are at a temperature above ambient. Incertain embodiments, burnishing is performed with the substrate 28 andthe fused pattern of metal 74 are approximately at ambient temperature.In certain embodiments, the pressure applied by the contact elementcreates heat in the contact area of the fused pattern of metal 74. Incertain embodiments, the relative motion of the contact element relativeto the substrate creates friction and therefore heat in the contact areaof the fused pattern of metal 74.

FIGS. 15A-15B depict another method of printing a circuit elementaccording to certain aspects of the present disclosure. FIG. 15A depictsa drop 150 of a first mixture that comprises nanoparticles 152 thatcomprise copper with at least a partial coating of a surfactant (notvisible in FIG. 15A and 15B) ejected by a printer (not shown in FIG. 15Aor 15B) similar to that of FIG. 2. In certain embodiments, the mixturecomprises a carrier. In certain embodiments, the carrier is a liquid. Incertain embodiments, the nanoparticles 152 are less than 50 nanometersin diameter. In certain embodiments, the nanoparticles are less than 20nanometers in diameter. In certain embodiments, the nanoparticles areless than 10 nanometers in diameter. In certain embodiments, thenanoparticles are in the range of 1-7 nanometers in diameter. In certainembodiments, the nanoparticles are in the range of 3-5 nanometers indiameter. The drops 150 strike a substrate 28 and form a layer 154 ofunfused nanoparticles 152.

FIG. 15B shows a drop 160 of a second mixture that comprises adispersant that is configured to displace the surfactants of thenanoparticles 152. The combination of the chemical action of thedispersant and the kinetic energy provided by the moving drop 160striking the layer 154 forces the unfused nanoparticles 152 in layer 154into contact with each other with sufficient pressure to at leastpartially fuse the nanoparticles into a fused layer 156.

In summary, circuit elements that are created by fusing coppernanoparticles at temperatures below 70° C. and the method of forming thecircuit elements are disclosed. Copper nanoparticles having diameters ofless than 20 nanometers, and preferably less than 10 nanometers, andmore preferably less than 4 nanometers, have properties that are notlinear extensions of their bulk properties. Copper nanoparticles mostpreferably having a diameter of less than 4 nanometers are fused by theimpact of drops of a mixture containing the copper nanoparticles thatare sprayed onto a substrate. Copper nanoparticles having a diameterpreferably less than 10 nanometers are fused by compression or heatingto a temperature of less than 70° C. of a mixture containing the coppernanoparticles.

The previous description is provided to enable a person of ordinaryskill in the art to practice the various aspects described herein. Whilethe foregoing has described what are considered to be the best modeand/or other examples, it is understood that various modifications tothese aspects will be readily apparent to those skilled in the art, andthe generic principles defined herein may be applied to other aspects.Thus, the claims are not intended to be limited to the aspects shownherein, but is to be accorded the full scope consistent with thelanguage claims, wherein reference to an element in the singular is notintended to mean “one and only one” unless specifically so stated, butrather “one or more.” Unless specifically stated otherwise, the terms “aset” and “some” refer to one or more. Pronouns in the masculine (e.g.,his) include the feminine and neuter gender (e.g., her and its) and viceversa. Headings and subheadings, if any, are used for convenience onlyand do not limit the invention.

It is understood that the specific order or hierarchy of steps in theprocesses disclosed is an illustration of exemplary approaches. Basedupon design preferences, it is understood that the specific order orhierarchy of steps in the processes may be rearranged. Some of the stepsmay be performed simultaneously. The accompanying method claims presentelements of the various steps in a sample order, and are not meant to belimited to the specific order or hierarchy presented.

Terms such as “top,” “bottom,” “front,” “rear” and the like as used inthis disclosure should be understood as referring to an arbitrary frameof reference, rather than to the ordinary gravitational frame ofreference. Thus, a top surface, a bottom surface, a front surface, and arear surface may extend upwardly, downwardly, diagonally, orhorizontally in a gravitational frame of reference.

A phrase such as an “aspect” does not imply that such aspect isessential to the subject technology or that such aspect applies to allconfigurations of the subject technology. A disclosure relating to anaspect may apply to all configurations, or one or more configurations. Aphrase such as an aspect may refer to one or more aspects and viceversa. A phrase such as an “embodiment” does not imply that suchembodiment is essential to the subject technology or that suchembodiment applies to all configurations of the subject technology. Adisclosure relating to an embodiment may apply to all embodiments, orone or more embodiments. A phrase such an embodiment may refer to one ormore embodiments and vice versa.

The word “exemplary” is used herein to mean “serving as an example orillustration.” Any aspect or design described herein as “exemplary” isnot necessarily to be construed as preferred or advantageous over otheraspects or designs.

The term “optical” covers electromagnetic radiation from ultraviolet toinfrared, including wavelengths in the range of 10 nanometers to 1millimeter and includes, but is not limited to, light visible to thehuman eye, which covers the range of 380-760 nanometers.

All structural and functional equivalents to the elements of the variousaspects described throughout this disclosure that are known or latercome to be known to those of ordinary skill in the art are expresslyincorporated herein by reference and are intended to be encompassed bythe claims. Moreover, nothing disclosed herein is intended to bededicated to the public regardless of whether such disclosure isexplicitly recited in the claims. No claim element is to be construedunder the provisions of 35 U.S.C. §112, sixth paragraph, unless theelement is expressly recited using the phrase “means for” or, in thecase of a method claim, the element is recited using the phrase “stepfor.” Furthermore, to the extent that the term “include,” “have,” “with”or the like is used in the description or the claims, such term isintended to be inclusive in a manner similar to the term “comprise” as“comprise” is interpreted when employed as a transitional word in aclaim.

1. An ink adapted for forming conductive elements, the ink comprising: aplurality of nanoparticles that comprise copper, the nanoparticleshaving a diameter of less than 20 nanometers, each nanoparticlecomprising at least a partial coating of a surfactant configured toseparate adjacent nanoparticles; and a carrier.
 2. The ink of claim 1,wherein the nanoparticles have a diameter of less than 10 nanometers. 3.The ink of claim 2, wherein the nanoparticles have a diameter in therange of 1-7 nanometers.
 4. The ink of claim 3, wherein thenanoparticles have a diameter in the range of 3-5 nanometers.
 5. The inkof claim 1, wherein the nanoparticles are configured to fuse to oneanother when one nanoparticle strikes another nanoparticle with arelative velocity greater than 0.5 meter per second.
 6. The ink of claim5, wherein the nanoparticles are configured to fuse to one another whenone nanoparticle strikes another nanoparticle with a relative velocitygreater than 1.0 meter per second.
 7. The ink of claim 1, wherein thenanoparticles are configured to fuse to one another when onenanoparticle is pressed against another nanoparticle with aninterparticle pressure, considered as an interparticle force divided bya nominal cross-sectional area of a nanoparticle, that is less than 600kilopascals (kPa).
 8. The ink of claim 7, wherein the nanoparticles areconfigured to fuse to one another when the interparticle pressure isless than 5000 kilopascals (kPa).
 9. The ink of claim 8, wherein thenanoparticles are configured to fuse to one another when theinterparticle pressure is less than 14000 kilopascals (kPa).
 10. Acircuit assembly comprising: a substrate having a maximum exposuretemperature of less than 200° C.; and a first layer of formed metalcoupled to the substrate, the first layer of formed metal comprising atleast partially fused nanoparticles that comprise copper and had adiameter of less than 50 nanometers prior to being fused.
 11. Thecircuit assembly of claim 10, wherein the nanoparticles had a diameterof less than 20 nanometers prior to being fused.
 12. The circuitassembly of claim 11, wherein the nanoparticles had a diameter of lessthan 10 nanometers prior to being fused.
 13. The circuit assembly ofclaim 12, wherein the nanoparticles had a diameter had a diameter in therange of 1-7 nanometers prior to being fused.
 14. The circuit assemblyof claim 13, wherein the nanoparticles had a diameter had a diameter inthe range of 3-5 nanometers prior to being fused.
 15. The circuitassembly of claim 10, wherein the substrate is flexible.
 16. The circuitassembly of claim 10, wherein the substrate has a maximum exposuretemperature of less than 70° C.
 17. The circuit assembly of claim 10,further comprising: a first layer of a nonconductive material over atleast a portion of the first layer of formed metal; and a second layerof formed metal over the first nonconductive layer, the second layercomprising the at least partially fused nanoparticles.
 18. The circuitassembly of claim 17, further comprising at least one via thatpenetrates the first nonconductive layer and electrically connects thefirst and second metal layers, the via comprising fused nanoparticlesthat comprise copper.
 19. The circuit assembly of claim 10, wherein thefirst layer of formed metal forms at least a portion of a passive deviceselected from the set of a resistor, a capacitor, an inductor, and adiode.
 20. The circuit assembly of claim 10, wherein the first layer offormed metal forms at least a portion of a transistor.
 21. The circuitassembly of claim 10, wherein the first layer of formed metal forms atleast a portion of a battery.
 22. The circuit assembly of claim 10,further comprising discrete electrical components.
 23. Acircuit-printing device comprising: a sprayer configured to emit aplurality of drops of a mixture comprising nanoparticles that comprisecopper and have a diameter of less than 20 nanometers toward a substratewith sufficient velocity that the nanoparticles at least partially fusewith each other upon impact with the substrate and form a plurality ofdots on the substrate, wherein each dot comprises a layer of at leastpartially fused nanoparticles and overlapping dots are fused to eachother.
 24. The circuit-printing device of claim 23, wherein thenanoparticles have a diameter of less than 10 nanometers.
 25. Thecircuit-printing device of claim 24, wherein the nanoparticles have adiameter in the range of 1-7 nanometers.
 26. The circuit-printing deviceof claim 25, wherein the nanoparticles have a diameter in the range of3-5 nanometers.
 27. The circuit-printing device of claim 23, wherein thevelocity of the emitted drops is greater than 0.5 meter per second. 28.The circuit-printing device of claim 27, wherein the velocity of theemitted drops is greater than 1.0 meter per second.
 29. Thecircuit-printing device of claim 23, further configured to create aconductive pattern of metal without exposure of the substrate to atemperature greater than 200° C.
 30. The circuit-printing device ofclaim 29, further configured to create a conductive pattern of metalwithout exposure of the substrate to a temperature greater than 70° C.31. A method of creating a conductive element on a substrate, the methodcomprising the step of: spraying a plurality of drops of a mixturecomprising nanoparticles that comprise copper and have a diameter ofless than 20 nanometers toward a substrate with sufficient velocity thatthe nanoparticles at least partially fuse with each other upon impactwith the substrate and form a plurality of dots on the substrate,wherein each dot comprises a layer of fused nanoparticles andoverlapping dots are fused to each other.
 32. The method of claim 31,wherein the nanoparticles have a diameter of less than 10 nanometers.33. The method of claim 32, wherein the nanoparticles have a diameter inthe range of 1-7 nanometers.
 34. The method of claim 33, wherein thenanoparticles have a diameter in the range of 3-5 nanometers.
 35. Themethod of claim 31, wherein the temperature of the substrate remainswithin 20° C. of ambient temperature while the nanoparticles are fusingto each other.
 36. The method of claim 31, wherein the temperature ofthe substrate remains below 200° C. while the nanoparticles are fusingto each other.
 37. The method of claim 36, wherein the temperature ofthe substrate remains below 70° C. while the nanoparticles are fusing toeach other.
 38. The method of claim 31, wherein the substrate isflexible.
 39. The method of claim 31, further comprising the step ofapplying a coating of a protective material over the fusednanoparticles.
 40. The method of claim 31, wherein the drops are sprayedwith a velocity that is greater than 0.5 meter per second.
 41. Themethod of claim 40, wherein the drops are sprayed with a velocity thatis greater than 1.0 meter per second.
 42. A method of creating aconductive element on a substrate, the method comprising the steps of:applying a layer of a mixture comprising nanoparticles that comprisecopper and have a diameter of less than 20 nanometers over at least aportion of a surface of a substrate; and at least partially fusing thenanoparticles together in at least a portion of the mixture layer. 43.The method of claim 42, wherein the nanoparticles have a diameter ofless than 10 nanometers.
 44. The method of claim 43, wherein thenanoparticles have a diameter in the range of 1-7 nanometers.
 45. Themethod of claim 44, wherein the nanoparticles have a diameter in therange of 3-5 nanometers.
 46. The method of claim 42, wherein the step offusing comprises directing a beam of optical radiation onto a portion ofthe mixture layer.
 47. The method of claim 46, wherein the opticalradiation has a frequency band that is at least partially within theabsorption band of the nanoparticles.
 48. The method of claim 42,wherein the step of fusing comprises applying pressure to the mixturelayer.
 49. The method of claim 48, wherein the pressure is applied bycompressing the mixture layer with a form.
 50. The method of claim 48,wherein the pressure is applied by dynamically nanoinscribing a patternover the mixture layer.
 51. The method of claim 48, wherein the pressureis applied by burnishing at least a portion of the mixture layer with aroller.
 52. The method of claim 51, wherein burnishing comprisesrotating the roller such that a surface of the roller that is in contactwith the mixture has a velocity greater than zero with respect to thesubstrate.
 53. The method of claim 48, wherein the pressure is appliedby burnishing at least a portion of the mixture layer with a ball. 54.The method of claim 53, wherein: the step of applying comprises applyinga layer of the mixture to an inside surface of a cylindrical passagethrough the substrate; and the step of fusing comprises passing the ballthrough the passage.
 55. The method of claim 54, wherein: thecylindrical passage has an inside surface with an inside diameter; andthe ball has a diameter that is greater than or equal to the insidediameter of the cylindrical passage.
 56. The method of claim 54, whereinthe step of fusing comprises rotating the ball such that a surface ofthe ball that is in contact with the mixture has a velocity greater thanzero with respect to the inside surface of the cylindrical passage. 57.The method of claim 48, wherein: the step of applying comprisesproviding a quantity of the mixture onto the substrate and spreading themixture with a tool drawn across the substrate; and the application ofpressure is accomplished by pressing the tool against the substrate asthe tool is drawn across the substrate.
 58. The method of claim 42,further comprising the step of removing the unfused portion of themixture layer.
 59. The method of claim 42, wherein the step of applyingcomprises applying the mixture in a pattern.
 60. The method of claim 59,wherein the step of applying a patterned layer of the mixture comprisesforcing the mixture out of a nozzle onto the surface of the substrate.61. The method of claim 59, wherein the step of fusing comprisesilluminating the patterned mixture with a source of optical energy. 62.The method of claim 59, wherein the step of fusing comprises compressingthe patterned mixture against the substrate.
 63. The method of claim 59,wherein the step of fusing comprises heating the patterned mixture to atemperature that is above ambient and below 200° C.
 64. The method ofclaim 63, wherein the step of fusing comprises heating the patternedmixture to a temperature that is above ambient and below 70° C.
 65. Themethod of claim 64, wherein the step of applying a patterned layer ofthe mixture comprises applying the mixture to a transfer element andthen transferring the mixture from the transfer element to thesubstrate.
 66. The method of claim 65, wherein the step of fusing thenanoparticles together comprises pressing the transferred patternedmixture against the substrate.
 67. The method of claim 42, wherein thetemperature of the substrate remains below 200° C. while the conductiveelement is formed on the substrate.
 68. The method of claim 67, whereinwhile the conductive element is formed on the substrate 70° C. while theconductive element is formed on the substrate.
 69. The method of claim68, wherein the temperature of the substrate remains within 20° C. ofambient temperature while the conductive element is formed on thesubstrate.
 70. The method of claim 42, wherein the step of applyingcomprises spin-coating the substrate with the mixture.
 71. The method ofclaim 42, wherein the step of applying comprises spraying the mixtureonto the substrate.
 72. The method of claim 42, wherein the step ofapplying comprises drawing a squeegee across the substrate.
 73. Acircuit-printing device comprising: a first sprayer configured to emit aplurality of drops of a first mixture comprising nanoparticles thatcomprise copper and have a diameter of less than 50 nanometers, eachnanoparticle comprising at least a partial coating of a surfactantconfigured to separate adjacent nanoparticles, onto a substrate; and asecond sprayer configured to emit a plurality of drops of a secondmixture comprising a dispersant configured to displace at least aportion of the surfactant from at least a portion of the nanoparticlespreviously applied to the substrate upon impact with the appliednanoparticles such that at least a portion of the applied nanoparticlesat least partially fuse with each other.
 74. The circuit-printing deviceof claim 73, wherein the nanoparticles have a diameter of less than 20nanometers.
 75. The circuit-printing device of claim 74, wherein thenanoparticles have a diameter of less than 10 nanometers.
 76. Thecircuit-printing device of claim 75, wherein the nanoparticles have adiameter in the range of 1-7 nanometers.
 77. The circuit-printing deviceof claim 76, wherein the nanoparticles have a diameter in the range of3-5 nanometers.
 78. The circuit-printing device of claim 73, furtherconfigured to create a conductive pattern of metal without exposure ofthe substrate to a temperature greater than 200° C.
 79. Thecircuit-printing device of claim 78, further configured to create aconductive pattern of metal without exposure of the substrate to atemperature greater than 70° C.